I. Field of the Invention
The present invention relates generally to pyroelectric devices.
II. Description of Related Art
Pyroelectricity, or the pyroelectric effect, is a physical property of polar dielectric materials which undergo a spontaneous change in polarization of the material in response to a change in temperature. The spontaneous polarization is defined as the polarization in a pyroelectric material that spontaneously forms in the absence of an applied electric field. Pyroelectric materials are used in a variety of sensing and detecting applications. Notably, they have found wide-spread use as electromagnetic energy detectors and bolometers at various wavelengths, including infra-red (IR) detectors and imagers for motion sensing and night-vision applications. In the radio frequency (RF) and microwave bands, they are used for instrumentation-grade power level sensors and radio astronomy radiometers, and can also be used for the detection of millimeter-wave and THz frequencies.
The amount of change of polarization in response to a change in temperature
  (            ∂      P              ∂      T        )is often quantified as a vector p, known as the pyroelectric coefficient. Physically, the internal structure of the electric dipoles of the pyroelectric material becomes modified during a temperature change. This modification can include a reorientation of molecular dipoles in the case of polymeric materials or a displacement in the atomic arrangement of charged atoms in the case of ferroelectric crystals. These displacements in turn cause a change in the spontaneous polarization and subsequent change in the surface charge of the material.
A typical pyroelectric capacitor is formed by adding a top and a bottom electrode to a pyroelectric material. The pyroelectric capacitor is then connected to an external circuit so that a variation in the surface charge on the capacitor will generate a flow of current as that charge is redistributed between the top and bottom faces of the capacitor in response to temperature changes. This current is given by the formula ip=Ap(dT/dt) where A is the area of the capacitor, p is the pyroelectric coefficient, and (dT/dt) is the time rate of change of temperature.
In a pyroelectric material, the internal dipoles which form the polarization are typically broken up into small regions of homogenous polarization known as domains. In polar crystals, the orientation of these domains is restricted by the structure of the material since the atomic arrangements, and their displacements, responsible for the polarization are restricted by the bonding symmetry of the crystal. For example, in the case of barium titanate (BaTiO3), a ferroelectric oxide with a perovskite crystal structure, the polarization occurs from a shift in the location of the central Ti atom with respect to the oxygen octahedral cage (consisting of 6 oxygen atoms) surrounding the Ti atom. The atomic displacement of the Ti atom is restricted to orthogonal crystallographic directions (displacements towards one of the 6 surrounding oxygens). Subsequently, the polarization orientations are limited to these orthogonal directions.
In polycrystalline materials with random grain orientation only a component of each domain will be able to contribute to the overall measured polarization. Only those domains that are oriented normal to the capacitor face will be able to contribute their full component of polarization to the pyroelectric device. The pyroelectric properties are thus compromised and diminished by misaligned domains in these materials.
Additionally, stress and strain are critical factors in the physical properties of pyroelectric materials. In a typical pyroelectric thin film, the film is rigidly clamped to the supporting substrate. Strains in the film are then created from lattice and thermal mismatches with the substrate. For films on non-lattice matched substrates, thermal strain from differences in thermal expansion coefficients is the most prominent source of strain. Most of the useful pyroelectric materials, including Lead Zirconium Titanate (PZT) and Barium Strontium Titanate (BST) and their derivatives, will develop in-plane (biaxial) tension on the commonly used substrates in the microelectronics industry, including silicon and sapphire. In-plane tensile strains are known to diminish the out-of-plane pyroelectric coefficient of these materials. Therefore, typical thin-film pyroelectric devices will show reduced performance when integrated into common complimentary metal-oxide semiconductor (CMOS) processing. This is a limiting factor when attempting to integrate pyroelectric devices directly into semiconductor fabrication, such as manufacturing pyroelectric arrays directly on silicon-based read-out circuitry.